Method of fabricating micro-mirror switching device

ABSTRACT

Design of a micro-mirror switching device and its fabrication in single crystal silicon are described. The device is composed of three main elements: silicon mirror plate with metal-mirror, secondary actuator, and hinge/spring mechanism to integrate the mirror plate with the actuator. p-n junction is first formed on p-type silicon. Trenches are then etched in n-silicon to define the device element boundaries and filled with silicon dioxide. Three layers of sacrificial oxide and two structural poly-silicon layers are deposited and patterned to form device elements. Novel release processes, consisting of backside electrochemical etching in potassium-hydroxide, reactive ion etching to expose oxide-filled trenches from the bottom, and hydrofluoric acid etching of sacrificial oxide layers and oxide in silicon trenches, form the silicon blocks; those that are not attached to structural poly-silicon are sacrificed and those that are attached are left in place to hold together the switching device elements.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The present invention relates generally to the design and methodof fabricating micro-machined micro-actuators and micro-mirror switches;and more particularly to fabricate metallic mirrors on single crystalsilicon (SCS).

[0003] (2) Description of the Prior Art

[0004] After its first conception in the 80's as a display device, MEMS(micro electro-mechanical system) micro-mirrors have come a long way inrevolutionizing the data transport in communication networks. This areahas been of particular interest for people involved in MEMS, in the pastten years. Micro-mirror devices have applications in display devices asa pixel, scanner type of display device. Light beam steering usingmicro-mirrors is being exploited in optical communication networks. MEMSbased optical cross connect devices are being used in long haul networksto select and switch the light data signals without converting them toelectronic domain. A large number of micro-mirror designs and mechanismshave been proposed. Surface micro machining is the most commonly usedmethod for fabricating micro-mirror devices. Thin films such aspoly-silicon, silicon dioxide, silicon nitride, and metal films such asaluminum, gold, chromium, and titanium have been used to developmicro-mirror devices for various applications. Some other researchershave used silicon-on-oxide MEMS, deep RIE SCREAM process, and in somecases backside aqueous potassium hydroxide etching to fabricatemicro-mirror devices. In most devices, torsion springs or free hingestogether with comb-drive or gap closing electrostatic actuators havebeen used.

[0005] U.S Pat. No. 5,537,083 describes a micro mechanical filter havingplanar components and fabricated using integrated circuit microfabrication techniques. The mechanical coupling between input and outputtransducers includes planar fixtures, displacement of the electrodesproducing bending of the elements of the fixtures. Processes includedepositing electrical signal processing circuitry on a substrate,depositing interface components between signal processing circuitry anda mechanical filter in a first layer; depositing in a second layercomponents of the mechanical filter.

[0006] U.S. Pat. No. 5,999,303 describes an optical head utilizing amicro machined element in combination with a light source and a lens toread and write data onto a storage disk. A micro-machined element mayinclude a tethered steerable micro-machined mirror. A movement of themicro machined mirror alters a beam of laser light transmitted from thelight source to the optical head and a reflected light beam from thestorage disk.

[0007] U.S. Pat. No. 6,210,988 B1 relates to micro-electromechanicalsystems using silicon-germanium films. The process includes depositing asacrificial layer of silicon-germanium (SiGe) onto the substrate;depositing a structural layer of SiGe onto the sacrificial layer, wherethe germanium (Ge) content of the sacrificial layer is greater than theGe content of the structural layer; and removing at least a portion ofthe sacrificial layer.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is a primary object of the invention to describethe design of single crystal silicon micro-mirror switching device.

[0009] Another object of the invention is to describe the design ofmicro-mirror switching device elements, mirror plate, secondaryactuator, and spring/hinge mechanism.

[0010] A further object of the invention is to provide a method forforming the MEMS micro-mirror switching device.

[0011] Yet another object of the invention is to provide methods forforming the elements of micro mirror-switching device, micro-mirror,actuator, and hinge/spring mechanism.

[0012] Another object of the invention is a method for implementing theangular amplification design using silicon as both structural andsacrificial layers.

[0013] In accordance with these objectives, design and a method offabricating the said micro-mirror switches are described. The switch hasthree elements: single crystal silicon (SCS) mirror plate, secondaryactuator, and hinge/spring mechanism to integrate the mirror plate withsecondary actuators. The actuator moves the micro-mirror through springand hinge mechanism. The actuator in this invention is of theelectrostatic type or of the thermal type. A p-n junction is firstformed on p-type silicon by growing n-type silicon on p-type wafers.Trenches are then formed to define the closed loop rectangularboundaries for micro-mirror plate, actuator electrodes, and othercomponents to be fabricated in SCS. After filling the trenches withsilicon dioxide, the wafer surface is planarized using chemicalmechanical polishing or other etch-back processes. The planarized waferis then used to fabricate top actuator electrode, interconnects, metalpads etc. Next, a sacrificial silicon dioxide layer-1 is deposited andpatterned. Protective layers of silicon nitride and poly-silicon arethen deposited and patterned. Next, another sacrificial silicon dioxidelayer-2 is deposited on which a layer of structural poly silicon layer-1is formed and patterned. This step is followed by the deposition ofanother sacrificial silicon dioxide layer-3 that is patterned.Structural poly-silicon-2 layer is then deposited and patterned. In thenext step, mirror metal is deposited and patterned to form mirror.Device formation is completed using the release processes to selectivelycut desired structures in single crystal silicon, while sacrificingremaining SCS blocks.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The objects, advantages, and details of fabricating the SCS micromirror-switching device according the design concept of this inventionwill be more clearly understood from the following detailed descriptiontaken in conjunction with the accompanying drawings.

[0015]FIG. 1 shows the top view of the switching device, with majorcomponents indicated by arrows.

[0016]FIG. 2 shows the top view of free hinge only design of the hingemechanism for implementing angular amplification.

[0017]FIG. 3 shows the top view of flexure spring and free hinge designfor implementing angular amplification.

[0018]FIG. 4 shows the top view of torsion spring and flexure springdesign for implementing angular amplification.

[0019] Due to the asymmetry of the device layout, the process isdescribed hereafter using two sets of cross-sections (shown by arrows inFIG. 1), each cross-section shown at various steps of the processingsequence. Each of the process steps cannot be shown on both thecross-sectional views, although the wafer sees all the process steps inentirety.

[0020]FIGS. 5a through 5 h are cross-sectional views of the device alongaxis 5A shown in FIG. 1.

[0021]FIGS. 6a through 6 h are cross-sectional views of the device alongaxis 6B shown in FIG. 1.

[0022]FIG. 5a and 6 a are cross-sectional views of the device after deeptrenches are etched, filled with silicon dioxide and planarized;sacrificial silicon dioxide layer-1 (sac-1) deposition and etching.

[0023]FIG. 5b is a cross-sectional view of the device after nitride andpoly-silicon protection layers are deposited and etched; sac-2 and sac-3oxide deposition.

[0024]FIG. 6b is a cross-sectional view of the device after nitride andpoly-silicon protection layers are deposited and etched; sac-2 and sac-3oxide deposition; structural poly-silicon layer deposition and etching.

[0025]FIG. 5c and 6 c are cross-sectional views of the device afterpattern etching of sac-2 and sac-3 oxides.

[0026]FIG. 5d is a cross-sectional view of the device after structuralpoly-silicon layer-2 deposition and pattern etching.

[0027]FIG. 6d is a cross-sectional view of the device after structuralpoly-silicon layer-2 deposition and pattern etching; mirror metaldeposition and etching.

[0028]FIGS. 5e and 6 e are cross-sectional views of the device after KOHetching of backside silicon.

[0029]FIGS. 5f and 6 f are cross-sectional views of the device afterreactive ion etching of silicon to expose silicon dioxide-filledtrenches from the bottom.

[0030]FIGS. 5g and 6 g are cross-sectional views of the device aftersacrificial etching of silicon dioxide through the trenches and fromtop.

[0031]FIG. 5h is a cross-sectional view of the final structure after theoxide and some of the silicon blocks are sacrificially removed, showinglower fixed silicon electrode, landing pad and silicon hinge parts.

[0032]FIG. 6h is a cross-sectional view of the final structure after theoxide and some of the silicon blocks are sacrificially removed, showinghinges, hinge bar, and mirror.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The top view of the micro-mirror switching device withmicro-mirror region 10, secondary actuator region 12, and hinge & springregion 14, is shown in FIG. 1. The hinge and spring element integratesthe micro-mirror and the actuator elements. Electrostatic actuatorconsisting of upper cantilever-type moving poly-silicon electrode andlower stationary silicon electrode moves the mirror through hinge andspring mechanism. Several combinations or hinge and spring elements areused: free hinge and lever arm (not shown); free hinge only combination(FIG. 2); flexure spring (not shown); flexure spring and free hinge(FIG. 3); free hinge and flexure spring (not shown); torsion spring andflexure spring (FIG. 4). Integration scheme according to the presentinvention implements the angular amplification concept for achievinglarge angular displacements for micro-mirrors with smaller bending ofsecondary cantilever-type actuators.

[0034]FIGS. 5a through 5 h and 6 a through 6 h describe the processsteps for forming the micro-mirror switching device. All the elements ofthe device are formed simultaneously during the described process steps.

[0035]FIG. 5a and FIG. 6a are cross-sectional views of the device alongaxis 5A and axis 6B respectively (shown by dashed lines 5A and 6B inFIG. 1) after several process steps as described herein is completed. Ap-n junction is first formed on p-type silicon 20. n-type silicon 22 isthen formed either by diffusion or alternatively by depositing epitaxialsilicon film. The depth of p-n junction is kept approximately at about8-10 μm. Trenches 24, approximately about 1-3 μm wide and about 4-8 μmdeep, are etched in silicon with a deep reactive ion etching process,known in prior art (U.S. Pat. No. 5,501,893). The said trenches definethe close-loop rectangular boundaries of the various regions 10, 12, and14 (shown in FIG. 1). Silicon dioxide 26 is then filled within thetrenches and the wafer surface is planarized using plasma etch back orchemical mechanical polishing methods, known in prior art. Silicondioxide 28, later used as sacrificial layer-1 (sac-1) is deposited onthe said planar surfaces and patterned. Resulting structures are shownin FIGS. 5a and 6 a.

[0036] Nitride 30 and poly-silicon 32 films, both used to protect fieldarea oxide, are deposited next and patterned. Sacrificial silicondioxide layer-2 (sac-2) 34 is then deposited but not patterned, as shownin FIGS. 5b and 6 b. Structural poly-silicon layer-1 (poly-1) 36 is thendeposited and patterned (shown only in FIG. 6b) to partly definespring/hinge elements.

[0037] Next , sacrificial silicon dioxide layer-3 (sac-3) 38 isdeposited and patterned as shown in FIGS. 5c and 6 c.

[0038] Structural poly-silicon layer-2 (poly-2) 40 is deposited next andpatterned, as shown in FIGS. 5d and 6 d. Poly silicon actuator andhinge/spring mechanism are fully defined at this process step.Mirror-metal is then deposited and the metal patterned to form themicro-mirror 42 as shown in FIG. 6d (only). At this step, all of thecomponents are fabricated in single crystal silicon, but not separatedor released to form working device elements.

[0039] The following steps describe the release process to cut thedevice structures in single crystal silicon and form the individualelements:

[0040] i) Backside silicon 20 is etched electrochemically preferably inaqueous potassium hydroxide (KOH) solution to stop at the n-p junctionwith diffused n-type silicon 22 exposed. Other etching mediums can besubstituted for the preferred aqueous KOH solution. These alternatechemicals include aqueous solution of ethylene diamine, pyrocatechol,and/or TMAH-isopropenol. A silicon window is opened from the backside ofthe wafer in this step. The resulting structure is shown in FIGS. 5e and6 e.

[0041] ii) Next, backside n-type silicon 22 is etched within saidKOH-etched silicon window, using reactive ion etching (RIE) process inSF6 plasma, known in prior art, or using further KOH etching of siliconwith time control, such that the silicon dioxide-filled trenches 26 areexposed at the bottom as shown in FIGS. 5f and 6 f.

[0042] iii) Silicon dioxide-filled trenches 26 are then sacrificiallyetched in HF solution of concentration approximately 49% HF in water atroom temperature. In this step, silicon dioxide is removed both fromtrenches and from under the micro-machined structures. The resultingstructures are shown in FIGS. 5g and 6 g. The unsupported silicon blocks44 get removed during this sacrificial etching step, while the siliconblocks 46 attached to structural poly-silicon layers remain in the finaldevice. The final device structures are shown in FIGS. 5h and 6 g/6 h.In FIG. 5h, lower fixed silicon electrode 48, landing pad 50, springpart 52, and silicon/poly-silicon hinge 54 are shown. In FIG. 6h, mirrorplate 56, free hinge 58, and silicon/poly-silicon hinge bar 60 areshown. The actuator shown in FIG. 5h is of the electrostatic cantilevertype with supporting blocks 48 and 50.

[0043] When the actuator is of the thermal type, the cantilever would beformed of a bi-metal or generically of a bi-morph material; and themovement of the actuator takes place due to mismatch in the thermalcoefficient of expansion of the two metals. The bi-morph cantilever ischosen from materials composed of silicon dioxide, silicon nitride, polysilicon, and/or gold and the heater material is chosen from aluminum,titanium nitride, aluminum-titanium alloy, and/or poly silicon. In termsof the process to incorporate the thermal actuator in the saidmicro-mirror device, the silicon blocks 48 and 50 are also sacrificedduring etching. The remaining process steps are identical to those usedin incorporating the electrostatic type of actuator.

[0044] The advantages of the present invention of forming the MEMSmicro-mirror switching device over the prior art are as follows:

[0045] 1. Micro-mirror switches fabricated on single crystal siliconusing conventional semiconductor device methods are flatter andtherefore able to steer the light beam more precisely.

[0046] 2. The switching device is formed using both surface micromachining and bulk micro-machining methods, thereby retaining theadvantages of both methods.

[0047] 3. Electrostatic, cantilever design of poly-silicon actuatorallows higher angular rotation of the mirror with small displacement ofthe actuator tip.

[0048] 4. New angular amplification concept allows the use of minimumspace for forming the actuator in the device for large angular bending.

[0049] 5. The invention method of fabrication uses a unique process offorming and using oxide-filled trenches (which are later sacrificiallyetched) that allows silicon to be used as both sacrificial andstructural layers.

[0050] 6. A new release process is used where, after electrochemicaletching of a window in backside silicon, reactive ion etching is used toexpose SiO₂-filled trenches and sacrificing the oxide through trenchesand from the top.

[0051] While the preferred embodiment of the invention has beendescribed, it will be understood by those skilled in the art thatvarious modifications in form and details may be made without departingfrom the spirit and scope of the invention. Accordingly, otherembodiments are within the scope of the following claims:

What is claimed is:
 1. Single crystal silicon micro-mirror switchingdevice comprising of: single crystal silicon mirror plate, secondaryactuator, and hinge/spring mechanism, with all the elements integratedon a single crystal silicon substrate.
 2. Micro-mirror switching deviceaccording to claim 1, wherein said secondary actuator is made ofpoly-silicon.
 3. Micro-mirror switching device according to claim 1,wherein said hinge/spring mechanism is made of silicon
 4. Secondaryactuator in micro-mirror switching device comprising: upper siliconelectrode and lower single crystal silicon electrode, formed on singlecrystal silicon substrate.
 5. Secondary actuator according to claim 4,wherein said upper electrode is made of poly-silicon.
 6. Secondaryactuator according to claim 5, wherein said upper poly-silicon electrodeis of the electrostatic cantilever type for bending.
 7. Secondaryactuator according to claim 4, wherein said lower silicon electrode isof the fixed type.
 8. Angular amplification element in micro-mirrorswitching device comprising: secondary actuator and hinge and springmechanism, formed on single crystal silicon substrate.
 9. Angularamplification element according to claim 8, wherein said secondaryactuator is made of upper poly-silicon cantilever electrode and lowerfixed silicon electrode
 10. Hinge and spring mechanism in micro-mirrorswitching device comprising: hinge and/or spring, formed on singlecrystal silicon substrate.
 11. Micro-mirror switching device accordingto claim 10, wherein the hinge/spring mechanism includes: free hinge andliver arms, free hinge without liver arm, flexure springs, flexurespring and free hinge, free hinge and flexure spring, and/or torsionspring and flexure spring.
 12. A method of forming a single crystalmicro-mirror switching device comprising: forming n-type silicon onp-type silicon substrate; forming a p-n junction; forming deep trenchesin n-type silicon; filling said trenches with silicon dioxide andplanarizing said dioxide; depositing silicon dioxide layer-1 onplanarized surface and patterning; depositing protective nitride andpoly-silicon films over silicon dioxide layer-1 and patterning;depositing silicon dioxide layer-2 over patterned nitride andpoly-silicon; depositing poly-silicon layer-1 over silicon dioxidelayer-2 and patterning poly-1; depositing silicon dioxide layer-3 overpatterned poly-silicon layer-1 and patterning silicon dioxide layers-2and -3; depositing poly-silicon layer-2 over patterned silicon dioxidelayers-2 and -3 and patterning poly-2; forming metal mirror bydepositing metal on the wafer and patterning metal; forming the devicestructures with a release process.
 13. A method of forming a singlecrystal micro-mirror switching device according to claim 12, whereindepositing n-type silicon over p-type silicon wafers forms said p-njunction.
 14. A method of forming a single crystal micro-mirrorswitching device according to claim 13, wherein n-type silicon is formedby process of diffusion.
 15. A method of forming a single crystalmicro-mirror switching device according to claim 13, wherein n-typesilicon is formed by epitaxial deposition.
 16. A method of forming asingle crystal micro-mirror switching device according to claim 13,wherein p-n junction depth is approximately about 8-10 μm.
 17. A methodof forming a single crystal micro-mirror switching device according toclaim 12, wherein said trenches in n-type silicon are approximatelyabout 1-3 μm wide and approximately about 4-8 μm deep.
 18. A method offorming a single crystal micro-mirror switching device according toclaim 12, wherein said n-type silicon is both a sacrificial and astructural layer.
 19. A method of forming a single crystal micro-mirrorswitching device according to claim 12, wherein said silicon dioxidelayer-1, silicon dioxide layer-2 and silicon dioxide layer-3 are of thesacrificial type.
 20. A method of forming a single crystal micro-mirrorswitching device according to claim 12, wherein said protectivedielectric layers are silicon dioxide, silicon nitride and/or polysilicon.
 21. A method of forming a single crystal micro-mirror switchingdevice according to claim 12, wherein said poly silicon-1 and polysilicon-2 are of the structural type.
 22. A method of forming a singlecrystal micro-mirror switching device according to claim 12, whereinsaid mirror metal is composed of aluminum, chromium, titanium, silver,platinum, and/or gold.
 23. A method of forming a single crystalmicro-mirror switching device according to claim 12, wherein saidrelease process comprises: electrochemical etching of back-side siliconin a base solution, reactive ion etching silicon from backside of saidn-type silicon, and etching sacrificial silicon dioxide layers inhydrofluoric acid solutions.
 24. A release method according to claim 23,wherein said backside of silicon wafer is etched selectively to stop onsaid n-type silicon.
 25. A release method according to claim 23, whereinsaid backside of silicon wafer is etched electrochemically, and in thepreferred embodiment, in potassium hydroxide (KOH) of concentration, 35%KOH in water at 75 C.
 26. A release method according to claim 23,wherein n-type silicon is etched, in the preferred embodiment, from theunderside with reactive ion etching process in SF₆ plasma.
 27. A releasemethod according to claim 23, wherein n-type silicon is etched,alternatively, in aqueous KOH with time control.
 28. A release methodaccording to claim 23, wherein etching of said sacrificial silicondioxide layers is done in 49% aqueous hydrofluoric acid solution at roomtemperature.
 29. A release method to form structural components insingle crystal silicon switching device; the method comprising: etchingof backside of silicon wafer in a selective manner, etching of n-siliconin aqueous KOH from backside, reactive ion etching silicon from backsideof said n-type silicon, and etching of sacrificial silicon dioxidelayers.
 30. A method of forming a MEMS device with silicon as structuraland sacrificial layers comprising: forming n-type silicon on p-typesilicon substrate; forming a p-n junction over a silicon substrate;forming deep trenches in n-type silicon; filling said trenches withsilicon dioxide and planarizing said dioxide; depositing silicon dioxidelayer-1 on planarized surface and patterning; depositing protectivenitride and poly-silicon films over silicon dioxide layer-1 andpatterning; depositing silicon dioxide layer-2 over patterned nitrideand poly-silicon; depositing poly-silicon layer-1 over silicon dioxidelayer-2 and patterning poly-1; depositing silicon dioxide layer-3 overpatterned poly-silicon layer-1 and patterning silicon dioxide layers-2and -3; depositing poly-silicon layer-2 over patterned silicon dioxidelayers-2 and -3 and patterning poly-2; and forming the MEMS device witha release process.
 31. A method of forming a MEMS device according toclaim 30, wherein said release process comprises: electrochemicaletching of back-side silicon preferably in KOH of concentration 35% KOHin water at 75° C., reactive ion etching silicon from backside of saidn-type silicon, and etching sacrificial silicon dioxide layers inhydrofluoric acid solutions.
 32. A method of forming a single crystalmicro-mirror comprising: forming n-type silicon on p-type siliconsubstrate; forming a p-n junction over a silicon substrate; forming deeptrenches in n-type silicon; filling said trenches with silicon dioxideand planarizing said dioxide; depositing silicon dioxide layer-1 onplanarized surface and patterning; depositing protective nitride andpoly-silicon films over silicon dioxide layer-1 and patterning;depositing silicon dioxide layer-2 over patterned nitride andpoly-silicon; depositing poly-silicon layer-1 over silicon dioxidelayer-2 and patterning poly-1; depositing silicon dioxide layer-3 overpatterned poly-silicon layer-1 and patterning silicon dioxide layers-2and -3; depositing poly-silicon layer-2 over patterned silicon dioxidelayers-2 and -3 and patterning poly-2; forming metal mirror bydepositing metal on the wafer and patterning metal; and forming themicro-mirror element with a release process.
 33. A method of forming asingle crystal micro-mirror according to claim 32, wherein said mirrormetal is composed of aluminum, chromium, titanium, silver, platinum,and/or gold.
 34. A method of forming a single crystal micro-mirroraccording to claim 32, wherein said release process comprises:electrochemical etching of back-side silicon preferably in KOH ofconcentration 35% KOH in water at 75° C., reactive ion etching siliconfrom backside of said n-type silicon, and etching sacrificial silicondioxide layers in hydrofluoric acid solutions.
 35. A method of formingan electrostatic actuator comprising: forming n-type silicon on p-typesilicon substrate; forming a p-n junction over a silicon substrate;forming deep trenches in n-type silicon; filling said trenches withsilicon dioxide and planarizing said silicon dioxide; depositing silicondioxide layer-1 on planarized surface and patterning; depositingprotective nitride and poly-silicon films over silicon dioxide layer-1and patterning; depositing silicon dioxide layer-2 over patternednitride and poly-silicon; depositing poly-silicon layer-1 over silicondioxide layer-2 and patterning poly-1; depositing silicon dioxidelayer-3 over patterned poly-silicon layer-1 and patterning silicondioxide layers-2 and -3; depositing poly-silicon layer-2 over patternedsilicon dioxide layers-2 and -3 and patterning poly-2; and forming theactuator element with a release process.
 36. A method of forming anelectrostatic actuator according to claim 35 wherein said releaseprocess comprises: electrochemical etching of back-side siliconpreferably in KOH of concentration 35% KOH in water at 75 ° C. reactiveion etching silicon from backside of said n-type silicon, and etchingsacrificial silicon dioxide layers in hydrofluoric acid solutions.
 37. Amethod of forming hinge and spring element comprising: forming n-typesilicon on p-type silicon substrate, forming a p-n junction over asilicon substrate; forming deep trenches in n-type silicon; filling saidtrenches with silicon dioxide and planarizing said silicon dioxide;depositing silicon dioxide layer-1 on planarized surface and patterning;depositing protective nitride and poly-silicon films over silicondioxide layer-1 and patterning; depositing silicon dioxide layer-2 overpatterned nitride and poly-silicon; depositing poly-silicon layer-1 oversilicon dioxide layer-2 and patterning poly-1; depositing silicondioxide layer-3 over patterned poly-silicon layer-1 and patterningsilicon dioxide layers-2 and -3; depositing poly-silicon layer-2 overpatterned silicon dioxide layers-2 and -3 and patterning poly-2; andforming the hinge and spring element with a release process.
 38. Amethod of forming a hinge and spring mechanism according to claim 37,wherein said release process comprises: electrochemical etching ofback-side silicon preferably in KOH of concentration 35% KOH in water at75° C., reactive ion etching silicon from backside of said n-typesilicon, and etching sacrificial silicon dioxide layers in hydrofluoricacid solutions.
 39. A thermal actuator in micro-mirror switching devicecomprising of: actuator cantilever beam and heater element.
 40. Athermal actuator according to claim 39, wherein the actuator cantileverbeam is made of material composed of silicon dioxide, silicon nitride,poly silicon, aluminum and/or gold.
 41. A thermal actuator according toclaim 39, wherein the heater element is made of material composed ofaluminum, titanium nitride, aluminum-titanium alloy, and/or polysilicon.
 42. A method of forming a thermal actuator comprising: formingn-type silicon on p-type silicon substrate; forming a p-n junction overa silicon substrate; forming deep trenches in n-type silicon; fillingsaid trenches with silicon dioxide and planarizing said silicon dioxide;depositing silicon dioxide layer-1 on planarized surface and patterning;depositing protective nitride and poly-silicon films over silicondioxide layer-1 and patterning; depositing silicon dioxide layer-2 overpatterned nitride and poly-silicon; depositing poly-silicon layer-1 oversilicon dioxide layer-2 and patterning poly-1; depositing silicondioxide layer-3 over patterned poly-silicon layer-1 and patterningsilicon dioxide layers-2 and -3; depositing poly-silicon layer-2 overpatterned silicon dioxide layers-2 and -3 and patterning poly-2; andforming the actuator element with a release process.
 43. A method offorming a thermal actuator according to claim 42, wherein said releaseprocess comprises: electrochemical etching of back-side siliconpreferably in KOH of concentration 35% KOH in water at 75° C., reactiveion etching silicon from backside of said n-type silicon, and etchingsacrificial silicon dioxide layers in hydrofluoric acid solutions.